PrefixHeader.pch （全局.h）

You don't have to add xxx.h file when you use it.
You can just put it in PrefixHeader.pch then you can use xxx.h everywhere

1.create PrefixHeader.pch
File->new->file->other :pch file

2.set up
Build settings ->prefix header :PrefixHeader.pch (Required)
Build settings ->precompile prefix header :Yes (Optional)